Lowest unoccupied molecular orbital 1.3- butadiene

Figure 14.12 Ultraviolet excitation of 1,3-butadiene results in the promotion of an electron from i / , the highest occupied molecular orbital (HOMO), to 1//3, the lowest unoccupied molecular orbital (LUMO).

Theoretical calculations have been an important means of rationalizing the electronic course of hetero-Diels-Alder and related pericylic reactions for the formation of 1,2-thiazines 25 and 26. MOP AC 93 PM3 calculations have been used to deduce the regioselectivity of [4-1-2] cycloaddition reactions involving thiazinylium perchlorate 27 (Scheme 1) <1999TL1505>. Due to the higher lowest unoccupied molecular orbital (LUMO) coefficient at C-6 compared to N-2, the C-6 and S-1 behave preferentially as the dienophile double bond in cycloaddition reactions of this substrate with butadienes 28. 

According to the frontier orbital theory,525 electron-withdrawing substituents lower the energies of the lowest unoccupied molecular orbital (LUMO) of the di-enophile thereby decreasing the highets occupied molecular orbital (HOMO)-LUMO energy difference and the activation energy of the reaction. 1,3-Butadiene itself is sufficiently electron-rich to participate in cycloaddition. Other frequently used dienes are methyl-substituted butadienes, cyclopentadiene, 1,3-cyclohexa-diene, and 1,2-dimethylenecyclohexane. 

These theories assert that the pathway of a chemical reaction accessible to a compound is controlled by its highest occupied molecular orbital (HOMO). For the thermal reaction of butadiene, which is commonly called ground-state chemistry, the HOMO is 2 and lowest unoccupied molecular orbital (LUMO) is photochemical reaction of butadiene, which is known to be excited-state chemistry, the HOMO is 1//3 (Fig. 3.5.6). 

The orbital in ethylene that receives these electrons is the lowest-energy orbital available, the Lowest Unoccupied Molecular Orbital (LUMO). In ethylene, the LUMO is the tt antibonding orbital. If the electrons in the HOMO of butadiene can flow smoothly into the LUMO of ethylene, a concerted reaction can take place. 

Ultraviolet (UV) spectroscopy is a method cd structure determination applicable specifically to conjugated systems. When a coi jugated molecule is irradiated with ultraviolet light, ener gy absorption occurs and a it electron is promoted from the highest occupied molecular orbital tHOMOl to the lowest unoccupied molecular orbital (L.UMO1. For I,3< butadiene, radiation of 217 nm ia required. As a general rule, the... 

On irradiation with ultraviolet light ihp), 1,3-butadiene absorbs energy and a tt electron is promoted from the highest occupied molecular orbital, or HOMO, to the lowest unoccupied molecular orbital, or LUMO. Since the electron is promoted from a bonding tt molecular orbital to an antibonding it molecular orbital, we call this a tt —> tt excitation (read as pi to pi star ). The energy gap between the HOMO and the LUMO... 

The tt-M.O. of butadiene are shown in Fig. 18. As the symmetry of the complexes in which butadiene is bound varies and is often very low, we will refer to the 77-orbi-tals as 771, 7t2, 773 and 774. The two lowest orbitals 77i and n2 in free butadiene are occupied, and formally act as donor orbitals on complexing. The lowest unoccupied molecular orbital, L.U.M.O., 7r3, is generally accepted to be the important orbital for back donation. The large majority of butadiene complexes show the distances C — C2 and C3— C4 to be longer and C2—C3 to be shorter in the bound ligand than in the free molecules. This is consistent with occupancy of 773 in the complex. 

Figure 24.1 shows the molecular orbitals (MOs) for both ethene and 1,3-butadiene and identifies the highest occupied molecular orbitals (the HOMOs) and the lowest unoccupied molecular orbitals (the LUMOs) for both compounds. The MOs shown include the p-orbitals on each carbon atom. How are these molecular orbitals constructed First, only the p-orbitals of the diene and the alkene are used because these orbitals are involved in the Diels-Alder reaction. An alkene has two jt-electrons, so it will have two n-molecrdar orbitals. The two orbitals cannot be of the same energy, and the two electrons will reside, spin paired (see Chapter 3, Section 3.1.2), in the lowest energy orbital. 

To analyze whether these reactions are allowed or forbidden, chemists focus on the frontier molecular orbitals of the reactants. The frontier molecular orbitals consist of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). The terms highest and lowest refer to the energy of the orbitals, and the terms occupied and unoccupied refer to whether the orbital is populated with two electrons or is empty. For example, the HOMO and LUMO of butadiene would be orbitals 2 and 3 in Figure 20.2, respectively. The HOMO and LUMO of ethylene are simply the tt and tt orbitals given in Figure 1.21. 

According to the frontier orbital theory, the orbitals that control these reactions are the aforementioned HOMO of one reactant and the LUMO (lowest unoccupied molecular orbital) of the other reactant. So, for this reaction, we have two possible scenarios interaction between HOMO ij/2 of butadiene and LUMO (p of ethylene or that between HOMO ti of ethylene and LUMO of butadiene. As the following illustration indicates, both possibilities lead to bonding overlap between the interacting orbitals of the reactants so the reaction is allowed, as we aU know. In addition, as fotmd by theory, between these two possible scenarios, we favor the interaction between the HOMO of the electron-rich reactant (butadiene in this case) with the LUMO of the electron-poor reactant (ethylene). 

Fig. 4.19 (A-D) Contour plots of the two highest occupied and the two lowest unoccupied molecular orbitals in the ground state of rram -butadiene ipi — ip4 in order of increasing energy). The small filled circles indicate the positions of the atoms. The contour lines give the amplitudes of the wavefunctions in a plane parallel to the plane of the tc system, and above that plane by the Bohr radius Solid lines indicate positive amplitudes dotted lines, negative. The wavefunctions...

Equivalent conclusions are drawn by analysis of the frontier orbitals involved in the cycloaddition. For most combinations of reactants, the appropriate orbitals are the highest occupied molecular orbital (HOMO) of the diene of butadiene) and the lowest unoccupied molecular orbital (LUMO) of the olefin of ethylene). Reaction then occurs by interaction of the HOMO and the LUMO, which can be seen from the illustration below to be symmetry allowed, since the orbitals have the appropriate symmetry ... 

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